Video: Rendering with alpha channels

We have a text animation we are happy with, and now we'd like to render it. If you recall, I have two layers in this animation: I have the text layer, and I have a background movie with the penguins. If I was to render this as a composite, I only need to render the RGB channels. I wouldn't need an alpha channel because the movie is filling the entire frame. However, let's say your client isn't sure of what the background movie should be, so they've asked you only to render the animated title.

Watch the Online Video Course After Effects Apprentice 06: Type and Music

3h 48mBeginnerApr 28, 2011Updated Nov 20, 2012

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One of the cornerstones of motion graphics is creating and animating type. In this course, Trish Meyer shows how to typeset titles professionally and create custom animations, as well as apply and modify the hundreds of text animation presets that After Effects ships with. Additionally, Chris Meyer shows how to add audio to projects, including spotting "hit points" to align keyframes and video action.

The After Effects Apprentice videos on lynda.com were created by Trish and Chris Meyer and are designed to be used on their own and as a companion to their book After Effects Apprentice. We are honored to host these tutorials in the lynda.com Online Training Library®.

Rendering with alpha channels

We have a text animation we are happy with, and now we'd like to render it.If you recall, I have two layers in this animation:I have the text layer, and I have a background movie with the penguins.If I was to render this as a composite, I only need to render the RGB channels.I wouldn't need an alpha channel because the movie is filling the entire frame.However, let's say your client isn't sure of what the background movie should be,so they've asked you only to render the animated title.

In order for them to composite this animation on another background, I need torender an alpha channel.I will turn on the transparency grid so you can see exactly what's transparentand also see that this title has a nice black drop shadow.I want to make sure that the shadow is composited smoothly on whateverbackground they add in the editing program.If you'd like to see what the alpha channel looks like, you can set the ShowChannels to Alpha Channel, and you can even RAM preview it.Wherever the alpha channel is white, the text will be fully opaque; where it'sblack will be fully transparent--and the drop shadow is semitransparent.

So that all looks good, I will return to the RGB channel.To render my composition, I can select Make Movie from the Composition menu, oruse the shortcut Command+M on Mac, Ctrl+M on Windows.When I do that, the Render Queue will open.It will add the composition to the Render Queue using whatever default templatesare set for Render Settings in the Output module.Because I have a Preference set under Output Preferences, it automaticallyuses the name of the composition for the file name.

If I click on the name of the file, I can decide where I would like to save it.In this case, it's the last folder I used.I can change the default name to something more useful, such asPPlayhouse_title_alpha, and then I will click Save. And before I click onthe Render button, I want to make sure I have my Render Settings and OutputModule set up correctly.The Render Setting default I am using is Best Settings.The only problem I see is that the Time Span is set to Work Area Only.

Now, that can be a little dangerous.If I had set the work area to preview only a small section of my comp, that'sall that would be rendered.So I want to make sure I set this to Length of Composition.Even better, I should change the Best Settings template so that it always renderswith the length of composition.The other settings are fine, but I did want to point out that if I wasrendering the background movie-- and remember I had separated the fields earlierin the Intrepid Footage dialog--I would want to set the Field Render pop-up to Lower Field First.

That way both fields from the source movie would be output in the final movie.Since I'm not rendering the background, I do have a choice.In this case I think I would like to have a progressive movie, so I will makesure Field Render is off.I will click OK, and let's look at the Output Module.I will click on the word 'Lossless' to open the Output Module settings.In this template, it's only saving the RGB channels, and the alpha channel isbeing pre-multiplied to give it a nice clean look.I will cancel out of this dialog and point out that there is another optioncalled Lossless with Alpha.

Now when I open the Output Module Settings, you can see the channels default tosaving RGB plus Alpha.The other option is to only save the alpha channel and that can be useful in some cases.The only problem with this template is that the color channel is stillpre-multiplied. For a professional editing system, the preferred format isactually straight. I am probably going to use the Straight (Unmatted) AlphaChannel option more often than Premultiplied,so I am going to cancel and show you how to do this change permanently.

Under the Edit menu, let's open Templates > Output Module.We will select Lossless with Alpha, and remember the default is topre-multiply the alpha channel.Since I might want to use this sometimes, I will change this to Premultiplied,and then I will duplicate it and I'll make a second version called Straight.And of course this version I have to edit and I will change it to Straight (Unmatted).By the way, if you are not sure of the differences between Straightand Premultiplied, I cover these in detail at the end of the basicanimation installment.

So this template will use the Straight (Unmatted) option.So I will click OK, and now that I have edited by templates, my pop-up will showtwo options for alpha: Straight or Premultiplied--and I want to pick the Straight option.Now I could click the Render button now, but I will make one more change.I am going to change the Post Render action so that the finished movie will beimported automatically into the project.That way I won't have to open it in the finder to check that everythingrendered correctly.Click OK and now I will select the Render button, and this will only take a second.

It's a pretty simple animation. (ring)There is the happy-happy joy-joy sound.Now there is two ways to find my movie.I can either twirl down the Output Module and click on the file path and thatwill open it in QuickTime Player in the Finder.But since I have asked it to automatically import, I can find it in the Projectpanel. If I open it in CS5 by just double-clicking, it will open it in theFootage panel with the alpha channel factored in,so the edges look nice and clean--and in particular the drop shadow is nice and soft.

But look what happens if I open it in QuickTime Player. To do that, I will pressthe Option key on Mac or Alt key on Windows when I double-click the movie.In QuickTime Player, that nice soft drop shadow is showing up as a hard, ugly black shadow.Well, that's because QuickTime Player is not factoring in the alpha channel.It's only showing me the RGB channels.So it's not factoring in the transparency for the shadow in the RGB channels.This is actually a good thing and nothing to be worried about.

The only time this might be a problem is if you deliver a job and the clientopens it in QuickTime Player; you may get a panicked phone call. Just tell themto import it into their editing program, use the alpha channel, and everything willlook nice and clean.Of course if you want to email a quick proof, be sure to render with apremultiplied alpha.That way they can view it on a web site or in QuickTime Player and it will look good.Well, I hope that helps you understand how to render with an alpha channel.If you take a few minutes and edit the default templates for render settings andoutput modules, you will find the next time you make a movie, it will go a lotfaster because you have all the templates available.

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Q: This course was updated on 11/20/2012. What changed?

A: We have added four new movies to the end of Chapter 8, "Working With Audio." All four of these movies (Spotting dialog, Timing dialog to music, Mixing audio, and Refinements) apply to all versions covered by the course. In addition, there are new sets of exercise files designed for After Effects CS5.5 and After Effects CS6 and a companion movie that shows our premium subscribers how to use the exercise files.

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